This invention relates to an improved method and device for cooling and freezing a food or other item which is carried through the device on a belt or other moving substrate. More specifically, this invention relates to a device wherein a liquid and gas-phase cryogens are used to cool and/or freeze food items that are moved through the device on a belt. The transfer of heat from the food item to the cryogen is maximized through the use of a process in which liquid cryogen is sprayed into a stream of gaseous cryogen which is circulated around the food item while also using a novel impingement plate to create a stream of cryogen. A novel design of the device increases the heat transferred from the food items to the cryogen.
Commercial freezers typically rely on the transfer of heat from a food product that is to be chilled or frozen by using a fan or blower which is situated near a conveyer upon which the food is being carried. The food product entering the freezer has a boundary layer of air surrounding it which insulates the food product in the surrounding atmosphere. Traditional freezers have employed blowers that generate currents of cooling vapor in many directions so that a significant portion of the vapor does not contact the food product in a perpendicular direction. Under these conditions the vapor which does contact the food product often does not possess sufficient energy to substantially reduce the boundary layer around the surface of the food product. There is, therefore, a need to generate directed jets of cooling vapor so as to disturb the boundary layer.
U.S. Pat. No. 4,479,776 to Smith discloses an apparatus using a plurality of vertical tubes to provide a unidirectional air flow toward the food product.
U.S. Pat. No. 4,626,661 to Henke discloses the use of a plurality of nozzles along the pathway of a food product for delivering discrete jets of unidirectional cooling air.
The use of tubes or slots to direct air in a cooling or freezing device has met with only limited success due to the build-up of condensation or ice in the tubes or slots which quickly reduces the efficacy of the devices.
U.S. Pat. No. 5,487,908 to Appolonia et al. discloses a method and device for heating or cooling a food product on a moving substrate in which a continuous channel traversing at least a major portion of the width of the moving substrate converts multi-directional flow into unidirectional flow. Such a device suffers, however, from having such an increased rate of flow that the food products become entrained in the flow and controlled processing of the food item through the device becomes difficult.
Increasing the velocity of the stream of cryogenic vapor which impinges the food item will increase the average heat transfer coefficient in a linear manner. At a certain point, however, unless the impingement stream is carefully controlled the velocity may also be sufficient to damage the food product or to carry the food product off the conveyor and into undesirable locations elsewhere in the freezer.
Overall heat transfer rates are dependent on local heat transfer coefficients, i.e., the amount of heat transferred from the food products to the cryogen is dependent on the rate of heat transfer locally between the cryogen and the food item. Local heat transfer rate can be changed by controlling the distance from the source of impingement jets to the food product, the velocity of the impingement jets, the turbulence in the jet and the efficiency of the flow of cryogen.
A need remains, therefore, for a device which can rapidly chill and/or freeze a food item while reducing the amount of cryogen needed by extracting the maximum cooling effect from a given amount of cryogen. The device must also be capable of transporting food from an inlet to an outlet without damaging the food product. Additionally, the device must be able to control the throughput of food items and must be resistant to the freezing and plugging of internal components by snow and ice build-up.
Accordingly, the present invention increases the amount of heat transferred from an item, particularly a food product, to a cryogen by generating impingement jets capable of breaking through the thermal boundary layer of the product, but which are not capable of damaging the product.
Furthermore, the present invention provides a jet of cryogenic gas to impinge the surface food products without causing the food products to become entrained in the impingement jet.
Furthermore, the present invention provides an efficient path for recirculation of cryogenic gases back to the blower means so as to increase the efficiency of the freezer.
Another advantage of the present invention is to provide a modular design which can be adapted to provide a plurality of solutions for food processing requirements.
A further advantage of the present invention is that the connection of the modules provides for continual impingement of cryogenic gas on food items from their entrance into the freezer apparatus until exiting.
An additional advantage of the present invention is the reduction in the dehydration of the food items which is accomplished through the immediate freezing of the exterior of the product upon entry into the apparatus.
A still further advantage of the present invention is the consistent cooling and or freezing of items across the width of the belt upon which the food items travel.
In one embodiment of the present invention a modular food chilling and/or freezing apparatus is provided which comprises an entrance module, an exit module and one or more intermediate modules. Each module contains a section of belt upon which food is transported. Each module contains an impinger which enables high velocity jets of cryogenic gas to impinge the upper and lower surfaces of the food items. The impinger may be a plate having a specific configuration of rounded or chamfered holes. In another embodiment a series of channels is used. A sprayer is provided in one or more modules in order to entrain droplets of liquid cryogen in the jets of cryogenic gas.
In an embodiment of the present invention the interface between modules includes a baffle for controlling the pressure differential and transfer of cryogen between modules.
In a further embodiment a pneumatically actuated ball valve vibrator modules used to remove the build-up of snow and ice from impingement plates.
In a still further embodiment of the present invention a hydraulic system is used to provide easy access to the interior of the apparatus.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.